The present invention relates to a printer for forming an image on a continuous-form recording sheet using an electrophotographic method.
There have been known image forming apparatuses using a so-called electrophotographic method such an electronic copying machine, in which an electrostatic latent image is formed by exposing a photoreceptor on the surface of a charged photoconductive drum. Toner is adhered to the latent image for development, transferred to a recording sheet and fixed by a fixing unit.
One of these is a laser beam printer which provides a hard copy of image information by scanning and exposing a photoconductive drum charged by laser beams. The beams are modulated based on image information such as figures, characters and the like, using the copy process of the above electrophotographic method.
The laser beam printer is very useful, because it can be widely used in such a manner that it prints figures of information received by an image reading unit such as an image scanner or can be as used an output terminal of a facsimile machine, and it can to output information at a high speed.
In general, such a laser beam printer conventionally comprises an existing electronic copying machine as a base unit thereof and uses so-called cut-type sheets cut into a predetermined size as a recording sheet. A so-called heat roll type fixing unit which comprises a pair of fixing rollers composed of a heat roller heated to a high temperature and a backup roller pressed there against are provided. A sheet on which an unfixed toner image corresponding to the image information is placed is caused to pass between the rollers so that it is heated and pressed, whereby the toner is melted and adhered on the recording sheet.
Incidentally, the electrophotographic method is such that the rotation of a photoconductive drum causes an exposed portion thereof to reach a transfer unit and a toner image is transferred onto the recording sheet fed at a speed identical with the peripheral speed of the photoconductive drum at the transfer unit. Thus, according to this process it is impossible to form images by intermittently interrupting the process due to the structure of the process.
Therefore, the laser beam printer is provided with a memory capable of storing at least one page of information, and when the one page of information is input therein, the printer outputs the information at one time.
It can of course be desired to use this laser beam printer as an output terminal of a computer. In this case, however, the use of a continuous-form sheet similar to that used in a conventional line printer is desired. The continuous-form sheet used in the conventional line printer is a so-called fan-folded sheet having feed holes and folded along perforated tear lines to enable the sheet to be simply cut off (hereinafter, simply referred to as a "continuous sheet").
When a continuous sheet is used to a laser beam printer using a heat roll fixing system, a length of the recording sheet feed path from a transfer unit to be the fixing position of the fixing unit must be set to substantially the same as a distance between the perforated tear lines of the continuous sheet in order to prevent such a disadvantage that when the laser beam printer stops (during a pause or because operation is completed), a page being subjected to a fixing action is stopped in the state that it is held between a pair of fixing rollers and semi-fixed toner in the process of being fixed remains caught between the pair of fixing rollers.
More specifically, since the continuous-sheet is finally cut off along the perforated tear lines for use, no image must be formed within a predetermined region in the vicinity of the perforated tear lines, and thus in a laser beam printer by which images are formed for each page, of the tear lines where no image is formed are arranged to be stopped in the state that it they correspond to the transfer unit. Consequently, when the length of the recording sheet feed path from the transfer unit to the fixing position is set to substantially the same as the distance between the perforated tear lines of the continuous sheet, the above disadvantage can be avoided, because the vicinity of the tear line where no image is formed is caused to correspond to the position of the fixing unit where fixing action is effected, when the laser beam printer stops.
As a result, however, a problem arises in that the printer as a whole is made unnecessarily larger in size to provide a necessary feed length of the recording sheet and a plurality of kinds of continuous sheets respectively having a different distance between perforated tear lines (i.e., a length of a page) cannot be interchangeably used in this laser beam printer. That is, continuous sheets having a different distance between perforated tear lines cannot be used.